WO1987002787A1

WO1987002787A1 - Coherent optical communications systems

Info

Publication number: WO1987002787A1
Application number: PCT/GB1986/000656
Authority: WO
Inventors: Leslie John Rysdale
Original assignee: Plessey Overseas Limited
Priority date: 1985-10-24
Filing date: 1986-10-23
Publication date: 1987-05-07
Also published as: GB2183055A; GB8526231D0; GB8625093D0; GB2183055B; EP0250461A1

Abstract

A control system for overcoming finite range limitation of state of polarisation (SOP) control devices. By providing additional SOP control devices, if one of the control devices near its operational limit, forcing certain combinations of devices to obey predetermined functions allows that device to be reset by a predetermined amount away from the limit in a manner which does not affect the output light SOP.

Description

COHERENT OPTICAL COMMUNICATIONS SYSTEMS The present invention relates to coherent optical communications systems and more particularly to such systems including polarisation correction devices. It is known that polarised light signals travelling in single mode optical fibres may undergo rotational effects resulting in a change in the state of polarisation (SOP) of the signals. Since receiver sensitivity is adversely affected by any SOP mismatch between an incoming signal and a local oscillator light signal, it is necessary for the SOP of either or both of the incoming light signal and the local oscillator light signal to be controlled.

Schemes including a variety of SOP control devices have been demonstrated in the past including electromagnetic fibre squeezers (See eg. Applied Physics Letters 1979, volume 35 pages 840 - 842, Polarisation stabilisation on single-mode fibre; R.Ulrich) and piezoelectric fibre squeezers (such as described by Mohr and Scholz in a paper entitled "Active polarisation stabilisation systems for use with coherent transmission systems or fibre optic sensors" 1983, 9th European Conference on Optical Communication pages 331 - 316). Other SOP control devices include rotatable phase plates (for example as disclosed in "Electronic Letters", volume 21, 1985 pages 52 - 53 in a letter entitled "Optical polarisation control using an optical heterodyne detection system" to Imai, Nosu and Yamaguchi) and rotatable fibre cranks (as disclosed by Okoshi, Furuya and Kikuchi in the same volume of "Electronic Letters" at pages 895-896 in a letter entitled "New Polarisation state control device: rotatable fibre cranks").

However, for high speed operation and immunity to vibration SOP control devices which have no moving parts are preferred. Devices of this kind include electro-optic crystals (such as that disclosed in "Electronic Letters" volume 16, 1980 at page 573 by Kubota, Oohara, Furuya and Suematsu in a letter entitled "Electro optical polarisation control on single-mode optical fibres") and faraday rotators (such as explained by Okoshi, Cheng and Kikuchi in "Electronic Letters" volume 21, 1985 at pages 787 - 788 in an article entitled "New polarisation-control scheme for optical heterodyne receiver using two faraday rotators"). The main problem with the kind of device mentioned in the preceding paragraph is that their operating ranges are limited. Thus for unrestricted SOP control it is necessary for the SOP control devices to be reset away from their limit of operation without disturbing the SOP of the light. It is an object of the present invention to provide a method of overcoming the operational limitations of the aforementioned SOP control devices and to provide apparatus using the method. According to the present invention there is provided a state of polarisation control system for use in a coherent optical communications system, the control system comprising a plurality of linear birefringent devices and at least one circular birefringent device, said birefringent devices co-operating to maintain the state of polarisation of light from a local oscillator substantially identical to the state of polarisation of a received light signal.

The method and apparatus in accordance with the invention will now be described by way of example only with reference to the accompanying drawings of which:-

Figure 1 is a schematic representation of a polarisation transformation configuration; and.

Figure 2 is a graphical representation of the relationship given by the apparatus of Figure 1.

The method proposed is based on the polarisation transformation configuration shown in Figure 1 to which reference is now made. Horizontally polarised light from the local oscillator (not shown) passes first through a linearly birefringent device 1 (for example an electro optic crystal) which has birefringence axes at ±45° to the horizontal and which causes a phase difference Ψ_χ radians between components of the emergent light field parallel and perpendicular to a birefringence axis. The resulting light then passes through a circularly birefringent device 2 (which may be a faraday rotator) with a phase difference Ψ_z induced between left and right circularly polarised components of the light field. Finally the light passes through two linearly birefringent devices 3 and 4 whose birefringence axes are at 0°/90° and ±45° to the horizontal and cause phase differences θ_y and θ_χ respectively. If Ψ_χ=0 and Ψ_z=π/2, the polarisation of the emergent light field can be set to any desired state by suitable choice of θ_χ and θ_y. If θ_χ =θ_χo and θy=θ_yo with Ψ _χ=0 and Ψ _z= ττ /2 , it can be shown that the SOP of the emergent light is unchanged if θ_χ is allowed to vary provided that θ_y and ψ_z satisfy the following relationships:-

if θ_yo ≠π/2 or 3π/2

Ψ_z-Cos^-1 (Sinθ_yoSin[θ_x-θ_xo])

y if θ_yo =π/2 or 3ττ/2

C

^v

The variation in Ψ_z and θ_y with θ_x-θ_xo is prlotted in Figure 2 for various values of θy_o.

Similarly, if θ_y if allowed to vary, the SOP can be kept constant by causing Ψ_z and Ψ_z to obey the following relationship:-

l_ y _{yo y} y_O

Ψ_x and Ψ_z repeat outside the range -π/2≤θ_xo ≤ 3π/2. In the automatic SOP control system of the invention θ_χ and θy are continually varied to maintain the local oscillator SOP identical to that of the signal and Ψ_χ is held at 0 with Ψ_z held at π/2. If, say, θ_χ nears the operational limit of the associated SOP control device, then by causing Ψ_z and θ_y to obey equation (1) above θ_χ may be moved back from the operational limit by an integral multiple of 2ττ radians without affecting the local oscillator SOP. The automatic control system then continue operation from this point. The values assumed by Ψ_z and θy as θ_x is moved are stored in a look-up table in the case of a microprocessor based system. Alternatively, since the problem of determining the required variation in Ψ_z and θy is identical with that of keeping the local oscillator SOP constant (and locked to the SOP of the signal) the control system itself may be switched over to controlling Ψ_z and θ_y as θ_x is backed off. In this latter case, if θ_yo= π/2 or 3 π/2, then, θ_y is switched between θy_o- ττ/2 and θy_o+ ττ/2 at θ_x-θ_xo = π2,3 π/2 etc. to ensure that Ψ_z follows the periodic variation shown in Figure 2. A similar strategy exists for resetting θ_y.

Accordingly the finite range limitation of the SOP control devices such as electro-optic crystals and/or faraday rotators may be overcome using the invention to provide an automatic SOP control system having endless polarisation control.

Figure 2 shows the relationship between θ_y, θ_yo, Ψ_z, θ_x and θ_xo given by the polarisation transformation configuration of Figure 1.

Claims

1. A state of polarisation control system for use in a coherent optical communications system, the control system comprising a plurality of linear birefringent devices and at least one circular birefringent device, said birefringent devices co-operating to maintain the state of polarisation of light from a local oscillator substantially identical to the state of polarisation of a received light signal.

2. A control system as claimed in claim 1 in which said at least one circular birefringent device comprises a faraday rotator.

3. A control system as claimed in claim 1 comprising means to compare operational integers of an associated state of polarisation control device with predetermined limit values of those integers and means arranged on detection of an integer approaching a limit value to cause the control device to move away from the limit values by an integral multiple of a value which does not alter the effective output state of polarisation.